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Repeated sequences (also known as repetitive elements, repeating units or repeats) are short or long patterns that occur in multiple copies throughout the genome.In many organisms, a significant fraction of the genomic DNA is repetitive, with over two-thirds of the sequence consisting of repetitive elements in humans. [1]
Highly repetitive DNA consists of short stretches of DNA that are repeated many times in tandem (one after the other). The repeat segments are usually between 2 bp and 10 bp but longer ones are known. Highly repetitive DNA is rare in prokaryotes but common in eukaryotes, especially those with large genomes. It is sometimes called satellite DNA.
A tract of repetitive DNA in which a motif of a few base pairs is tandemly repeated numerous times (e.g. 5 to 50 times) is referred to as microsatellite DNA. Thus direct repeat tandem sequences are a form of microsattelite DNA. The process of DNA mismatch repair plays a prominent role in the formation of direct trinucleotide repeat expansions. [2]
Genomic DNA is tightly and orderly packed in the process called DNA condensation, to fit the small available volumes of the cell. In eukaryotes, DNA is located in the cell nucleus, with small amounts in mitochondria and chloroplasts. In prokaryotes, the DNA is held within an irregularly shaped body in the cytoplasm called the nucleoid. [97]
Some repetitive sequences are ~50% G+C/A+T and thus have buoyant densities the same as bulk genomic DNA. These satellites are called "cryptic" satellites because they form a band hidden within the main band of genomic DNA. "Isopycnic" is another term used for cryptic satellites. [3]
DNA methylation is a major form of epigenetic control over gene expression and one of the most highly studied topics in epigenetics. During development, the human DNA methylation profile experiences dramatic changes. In early germ line cells, the genome has very low methylation levels. These low levels generally describe active genes.
In white blood cells, the gradual shortening of telomeric DNA has been shown to inversely correlate with ageing in several sample types. [11] Telomeres consist of repetitive DNA, with the hexanucleotide repeat motif TTAGGG in vertebrates. [citation needed] They are thus classified as minisatellites. Similarly, insects have shorter repeat motifs ...
In genetics, a minisatellite is a tract of repetitive DNA in which certain DNA motifs (ranging in length from 10–60 base pairs) are typically repeated two to several hundred times. [1] Minisatellites occur at more than 1,000 locations in the human genome and they are notable for their high mutation rate and high diversity in the population. [2]